Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility.  F-strings were released in Python 3.6.  Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.

test/python/unicode/wsgi.py

@@ -1,7 +1,7 @@
 def application(environ, start_response):
     temp_dir = environ.get('HTTP_TEMP_DIR')
 
-    with open(temp_dir + '/tempfile', 'w') as f:
+    with open(f'{temp_dir}/tempfile', 'w') as f:
         f.write('\u26a0\ufe0f')
 
     start_response('200', [('Content-Length', '0')])